Human extracellular calcium-sensing receptor (CaSR) is a G-protein coupled receptor that maintains Ca2+ homeostasis through the regulation of parathyroid hormone secretion. CaSR also plays important roles in biological processes unrelated to Ca2+ balance such as fetal development. Abnormalities in the CaSR gene are associated with a vareity of Ca2+ homeostatic disorders including potentially life threatening hypercalcaemic conditions. Altered expression of CaSR has also been implicated in other diseases including cancer and Alzheimer's disease. Cinacalcet, an allosteric modulator of CaSR, is used clinically to treat disorders of bone and mineral metabolism. Therefore, elucidating the structure of CaSR may assist the design of valuable therapeutic agents. CaSR functions as a disulfide-tethered homodimer. It activates a diverse array of signaling pathways in a ligand-specific manner. In addition to its principal agonist, extracellular Ca2+, CaSR responds to a variety of agonists, and its activity is regulated by postive and negative allosteric modulators. The first part of this proposal is aimed at understanding the mechanisms by which CaSR recognizes structurally distinct ligands while maintaining binding affinity and specificity. We propose to solve the crystal structures of CaSR ectodomain bound to different agonists and allosteric modulators. The second aim is to identify the conformational changes associated with receptor activation based on the structures of CaSR ectodomain in the resting and active states. We plan to measure the functional effects of mutations that are designed based on these structures to probe the ligand binding sites and homodimer interface. The third part of the proposed research is to elucidate the signal transduction mechanism of CaSR by solving the structures of full-length receptor and its complexes with downstream signaling molecules. A combination of structural and functional analysis of CaSR will advance our understanding of the molecular basis of extracellular Ca2+ sensing and the general activation mechanisms of GPCR dimers.